DESCRIPTION (provided by investigator): Prothrombinase (PTase) activity generates thrombin, a serine protease that cleaves fibrinogen, prompting the deposition of polymerized fibrin. In preliminary studies, we demonstrate upregulated PTase activity and fibrin deposition at sites of type 1 T helper (Th1) cell responses, and lymph nodes draining those sites. We also demonstrate that the Th1 cytokine IFN? stimulates expression of tissue factor and Fg12, genes that upregulate PTase activity, while the Th2 cytokine IL-4 stimulates expression of Protein S, a PTase antagonist. Thus, we hypothesize that Th1 and Th2 cells counter-regulate PTase activity. In Aim 1, we use transgenic T cell receptor (TCRtg) adoptive transfer models to quantitatively measure the capacities of individual T cell subsets and cytokines to stimulate or limit PTase activity, fibrin deposition and coagulant gene expression in vivo. We also use recently generated gene-targeted mice to definitively evaluate roles for Fg12 in Th1-associated PTase activity. Together, these studies will define mechanisms regulating immune-associated PTase activity. Having established that T cells regulate PTase activity, we hypothesize that products of PTase activity function in both normal and pathological immune responses. In preliminary studies, we demonstrate that the thrombin receptor PAR-I is expressed by murine T cells, regulated at sites of immunity, and influences T cell proliferation in vivo. We also establish that thrombin stimulates fibrinogen-dependent chemokine secretion and macrophage adhesion in vivo. In Aim 2, we use TCRtg models to define and distinguish roles for thrombin, PAR-1 and fibrin(ogen) in lymph node chemokine production, and other fundamental events accompanying acquired immunity. We also determine whether PTase products can stimulate or limit T cell responses when co-administered with antigen. Our overall goal is to define mechanisms by which PTase activity and its products influence immunity. Deciphering those mechanisms may lead to improved vaccines and/or treatments for pathologies characterized by fibrin deposition, including autoimmunity, allograft rejection, cancer and septic shock.